Cellular mobile radio telephone systems are well known. In such a system radiotelephone units utilize radio frequencies to communicate with relatively low power, relatively limited radiation base transceivers arranged in cellular pattern. This makes it necessary for the system to locate each radiotelephone unit and follow it enroute by "handing off" in-progress calls as the units move between cells. An overview of such systems can be found in the article "Cellular System Design: An Emerging Engineering Discipline" by J. F. Whitehead in IEEE Communications Magazine February 1986, Vol. 24 No. 2.
Such systems are described in more detail in the following U.S. patents to which attention is directed: U.S. Pat. No. 4,562,572 dated Dec. 31, 1985 to S. O. Goldman et al.; U.S. Pat. No. 4,096,440 dated June 1978 to Okasaka; U.S. Pat. No. 4,028,500 dated June 1977 to McClure et al.; U.S. Pat. No. 3,984,807 dated October 1976 to Haemmig; and U.S. Pat. No. 3,760,106 dated October 1973 to Monti. A signalling system that may find application in a cellular radio system is described in U.S. Pat. No. 4,210,780 dated July 1, 1980 by G. T. Hopkins et al. to which attention is also directed.
The prior art cellular radio systems are generally well engineered, pre-planned, and rigidly laid out systems. That is, the entire area to receive a cellular radio system is surveyed, sites are chosen for base stations, frequencies are allocated, etc. Once the planning for the system has been conducted, the system is built strictly according to the plan; the system may be expanded at a later date by adding "cells" at the periphery of the system, but the core of the system is virtually "cast in stone".
Alternatively, "cell splitting" may be used to increase the capacity of the system by a factor of 2 or 3. This is achieved by installing new base stations and specially engineered directional antennas at specific locations. The cell splitting method requires extensive planning and engineering effort.
The existing city-wide cellular radio systems may well be the most complex non-military radio communication systems ever put into operation. The cellular mobile telephone system has evolved through many years of research, from the early work on characteristics of various hexagonal cell configurations for frequency reuse, to the present architecture of an extensive centrally controlled system. Currently, cell planning is an elaborate engineering exercise that spans various phases of cellular design and operation. A typical city-wide cellular radio system is a centrally controlled one wherein certain parameters (like transmission power of base stations and the mobiles) are automatically controlled. Other parameters (such as channel allocations per cell) are fixed and only after a redesign of the system can they be changed by an operator.
An in-building communication system based on a scaled-down version of the city-wide cellular radio would be an overkill and operationally inefficient, as it would inherit unnecessarily all the engineering complexities of the cellular mobile radio. The present invention addresses a different need, and is required to operate in a different environment, than the city-wide cellular system.